An ignition coil for an internal combustion engine includes a primary coil, which is formed by winding an electrical wire around a primary spool, and a secondary coil, which is formed by winding an electrical wire around a secondary spool. The primary coil is arranged concentrically with respect to the secondary coil.
A center core made of a magnetic material is arranged on a radially inner side of the primary coil and the secondary coil. An outer core made of a magnetic material is arranged on the radially outer side of the primary coil and the secondary coil. In this structure, a magnetic path is defined through the center core and the outer core. A thermosetting resin such as epoxy resin is charged into a clearance formed in the ignition coil.
A stick-type ignition coil includes the primary coil, the secondary coil, the center core, the outer core, and the like. A coil portion of the stick-type ignition coil is arranged in a plughole of an engine. The coil portion of the stick-type ignition coil has an axial end provided with a connector case. The connector case includes a connector portion, through which electric power is supplied to the primary coil. The connector case further includes a flange portion, via which the ignition coil is fixed to an engine.
With a stick-type ignition coil in US 2005/0174206 A1 (JP-A-2005-260209), a connector case (control unit) is assembled to an axial end of a coil portion via a cylindrical-shaped fitting member (terminal assembly). The fitting member restricts leakage of epoxy resin charged in a clearance defined in the ignition coil. The fitting member is provided with a current-carrying terminal. The fitting member has an annular groove at which the fitting member is bendable. When the coil portion is fitted into the connector case with the fitting member therebetween, the fitting member is restricted from cracking by flexing the fitting member at the annular groove. The annular groove formed on the fitting member is not charged with resin. Therefore, the ignition coil is decreased in bending strength, even though the ignition coil is improved in quality of assembly.
In addition, the location, in which the fitting member is fitted to the connector case, overlaps the location, in which the fitting member is fitted to the outer core, with respect to the axial direction. The connector case is fitted to the outer periphery of the fitting member at the location, in which the outer core is fitted to the fitting member.
The coil portion is inserted into the plughole, so that the ignition coil is assembled to the engine. In this condition, when bending load acts on the ignition coil due to vibrations of the engine, stress is concentrated on the fitting member and the fitting portion.
The ignition coil is assembled with a spark plug, and the ignition coil and the spark plug are inserted into the plughole. In this condition, when the ignition coil, the spark plug, and the plughole are radially misaligned relative to each other, bending load is initially applied to the ignition coil. Bending load is further applied to the ignition coil due to vibration of the engine, and consequently, the ignition coil further decreases in bending strength.